This invention relates to clamps and, more particularly, to a clamp for securing wire screen to a traveling water screen used with water treatment intakes and the like.
Traveling water screens are used for screening water at the water intakes of power plants and also in many other types of processing centers which consume large amounts of water, both in industry and in government, where it is necessary to screen or filter debris and fish from large volumetric flows of water. A typical application is a power plant which requires a continuous, large volume stream of cooling water. Normally, the water for this purpose is supplied via a watercourse extending between the plant and a water source such as a lake, river, or ocean. Debris enters the watercourse with the water and must be filtered or screened out to prevent disruption of the operation of the plant or damage to equipment.
A number of traveling water screens are typically positioned in a concrete structure spanning the watercourse. Each traveling water screen commonly includes a series of screening panels attached between a pair of chains which are in turn supported on a pair of head sprockets at the top, and a pair of foot sprockets at the bottom, so that the screening panels form an endless band. The head sprockets are driven so that the screening panels lift out of the water on the upstream side, taking debris with them. Debris is then typically removed by a water spray near the top of the structure. The screen units then descend on the downstream side and pass around the foot sprockets to ascend again on the upstream side.
The screening panels carried by the heavy parallel chains each include a basket having a rectangular frame which is carried at its opposite ends by the spaced apart scrolled chains. The rectangular frames of the baskets each include end plates and transverse members extending between the chains. A generally rectangular panel of woven wire screen is fixed to the rectangular frames. In some such arrangements, as illustrated in U.S. Pat. No. 4,443,126, assigned to the assignee of the present invention, the margins or peripheral edges of the woven wire screens are bent 90.degree. to permit the screens to be fastened to the rectangular frames. In other prior art traveling water screen constructions, the basket frames may have four flat surfaces in a common plane and against which the margins of the flat screen panel are secured. In both prior art basket constructions described above, a rigid clamping bar typically overlies each screen margin, with bolts extending through holes in the bar and into aligned holes in the frame members, with the bolts tightened to clamp the screen margins between the clamping bars and frame member, and to secure the screen to the frame. All of the above mentioned parts are generally stainless steel or other corrosion resistant material, such as painted carbon steel.
U.S. Pat. No. 4,443,126 illustrates non-metallic clamping bars for securing screens to basket frames of a travelling water screen wherein the screens are bent 90.degree.. The non-metallic clamping bars comprise strips of a rigid polyvinyl chloride compound or other durable engineered plastic, and the clamping bars are secured to inner surfaces of the basket frames by bolts extending through the clamping bars, screens and frame. The synthetic material forming the clamping bars is non-corrosive and of less cost than metal strips.
For such traveling water screens used in water intakes of power plants or other industrial or municipal applications or systems which take in large volumes of water, the wire screen experiences considerable force from and in the direction of the flow of water and the screen is placed under substantial tensile forces. Failure of the screen can occur from metal fatigue caused by repeated or cyclical flexing of the screen due to the force of water on the screen. Metal fatigue tends to occur at areas where the clamps attach the wire screen to the frames, particularly where a hole in the screen does not align with the holes in the clamping bar and frame, leaving some slack in the screen. This increases the chances of screen fatigue because the cyclic bending from water pressure repeatedly bends the screen at the region of slack between the screen and the clamp, which can result in failure of the screen at the clamp margin.